1. Field of the Invention
This invention relates to a control signal detection method with calibration error and a subscriber unit for use therewith, and is applicable to, for instance, the digital cellular telephone system which is referred to as the GSM (Groupe Speciale Mobile) cellular system and that is standardized in Europe.
2. Description of the Related Art
Heretofore, in the GSM cellular system, a circuit is connected between a base station and mobile terminal equipment using the time division multiple access (TDMA) system, and voice data and others which have been coded are transmitted and received.
In the GSM cellular system, 124 broadcast channels are prepared as physical channels. Each broadcast channel is time-shared into 8 channels by the TDMA system. Logical channels are roughly separated into 2 channels, that is, an information channel and a control channel. The information channel is used to transmit coded voice data, etc., and the control channel is used to transmit various control signals.
The control channel includes a broadcast control channel (BCCH), a frequency correction channel (FCCH), a synchronization channel (SCH), a paging channel (PCH), a random access channel (RACH), attendant control channels, namely a slow associated control channel (SACCH), and a fast associated control channel (FACCH), etc.
In the GSM cellular system, the Gaussian filtered minimum shift keying (GMSK) is used as a modulating system, and each data is exposed to GMSK modulation and then transmitted. An example of the GMSK modulation has been disclosed in U.S. Pat. No. 5,131,008, assigned to Motorola Inc. In this patent, compensation of an in-phase signal and a quadrature signal is performed based on a coherent carrier signal.
By the way, to perform connection of a circuit by means of the time division multiple access, synchronization must be established between a base station and mobile terminal equipment. Therefore, after the electric power source has been energized, the mobile terminal equipment first detects the FCCH in the control channel which is transmitted by the base station, and then roughly realizes the initial synchronization on the basis of the detected FCCH and corrects the oscillating frequency of the local oscillator (hereinafter, this is referred to as the local oscillation frequency). However, the fine synchronism is established by the means of the SCH.
As shown in FIG. 1, one period of the control channel is comprised of 51 frames, and each frame is comprised of 8 slots. On the GSM cellular system, burst data in each slot are transmitted and received. On the control channel which has a constitution like this, the FCCH is inserted once (1 slot) within 10 frames. The FCCH is a control signal which is composed of continuous 0!s of the stated bits, and the data are not varied to 0! or to 1!, as contrasted to the other burst data. Therefore, when the received data of the case where the FCCH has been normally received is represented on a complex plane, the signal points are respectively rotated by 90.degree. in the same direction, as shown in FIG. 2. This is due to the fact that when the transmitted data are the series of the same values, signal points are respectively rotated by 90.degree. in the same direction on the same circle on a complex plane, in the case of GMSK modulation.
To detect such a FCCH, the mobile terminal equipment performs a calculation which is represented by the following expression: ##EQU1## with respect to the I,Q data (i.sub.k, q.sub.k) which is the received data, and obtains the correlation value. More specifically, the mobile terminal equipment is provided with a correlation value calculating circuit 1 shown in FIG. 3, so that the correlation value is obtained by this.
The received I,Q data (i.sub.k, q.sub.k) is first fed to a multiplier 2, and multiplied, by e.sup.-jp which is outputted from a numeric value generator 3 (where, P=.pi.k/2). The result of this multiplication is fed to an N-stage shift register 4, and shifted sequentially. N pieces of data which are output from the shift register 4 are respectively fed to an adder 5, and added. The result of the addition is fed into an absolute value circuit 6, so that the correlation value is obtained.
This obtained correlation value becomes a large value in the case where the received data is FCCH, and becomes a small value in the other cases, as shown in FIG. 4. Therefore, if the magnitude of the correlation values is continuously examined, the FCCH can be detected on the basis of such a fact that the correlation value have reached to the maximum value.
By the way, to obtain the correlation value, at first, standard adjustment (hereinafter, this is referred to as calibration) must be performed in such a manner that the center of the I,Q data comes to the position of the origin (0, 0) on the complex plane, prior to the start of receiving. Because, some signals are occasionally output owing to the characteristics of the circuits of the receiving system, even though it is in the non-receiving status. In order to compensate this, it is necessary that the center of the I,Q data be adjusted to the position of the origin on the complex plane. When such calibration is performed, heretofore, an analog calibration circuit shown in FIG. 5 is utilized.
In this calibration circuit 10, in the first place, at non-receiving time, switches SW1 and SW2 are brought into OFF state, and a switch SW3 is brought into ON state. By this, the difference of the I component and the -I component is charged in a capacitor C1, through a differential amplifier 11.
On the other hand, when receiving has been started, in the calibration circuit 10, the switches SW1 and SW2 are brought into ON state, and a switch SW3 is brought into OFF state. By this, compensation is performed with respect to the I component and the -I component, by the difference of non-receiving time, by means of current sources A1 to A4 and a differential amplifier 12, according to the electric charge which has been charged in the capacitor C1. In this connection, compensation is also performed in a similar manner with respect to the Q component.
By the way, in the GSM cellular system, only one slot of FCCH exists in 10 frames as stated above; therefore, continuous receiving operation of a long time (about 50 mS! which is corresponding to about 11 frames) is needed, until an FCCH is detected. In this case, such a phenomenon occurs that the compensation comes to be not performed correctly, because the electric charge that has been charged in the capacitor C1 of the abovementioned calibration circuit 10 is discharged. As a result, so-called calibration error occurs, which is such a phenomenon that the calibration which has been performed prior to the start of reception drifts in proportion as reception advances, as shown in FIG. 6.
Such a calibration error has bad influences; the probability of detection of FCCH is lowered, an error occurs in the estimate which is estimated when the drift of the local oscillation frequency (hereinafter, this is referred to as local oscillation frequency offset) would be calibrated, and others. Explaining it concretely, because a correlation value is used in order to detect FCCH as stated above, if a calibration error exists then the correlation value becomes small, therefore the probability of detection of FCCH is lowered.
When the local oscillation frequency offset is estimated, it is estimated on the basis of the quantity of phase drift (.DELTA..theta.) of the I,Q data, as shown in FIG. 7A. If the calibration has drifted, then a phase error (=.DELTA..theta.'-.DELTA..theta.) occurs as shown in FIG. 7B, so that an error occurs in the estimate of the local oscillation frequency offset as well.